1. Field of the Invention
This invention relates generally to glucagon-like peptide-1 (GLP-1), exendin-4 and their peptide analogs. The invention also relates to their uses in the treatment of diabetes and neurodegenerative conditions.
2. Background Art
Pancreatic beta cell dysfunction and the concomitant decrease in insulin production can result in diabetes mellitus. In type 1 diabetes, the beta cells are completely destroyed by the immune system, resulting in an absence of insulin producing cells (Physician's Guide to Insulin Dependent [Type I] Diabetes Mellitus: Diagnosis and Treatment, American Diabetes Association, 1988). In type 2 diabetes, the beta cells become progressively less efficient as the target tissues become resistant to the effects of insulin on glucose uptake. Thus, beta cells are absent in people with type 1 diabetes and are functionally impaired in people with type 2 diabetes.
Beta cell dysfunction currently is treated in several different ways. In the treatment of type 1 diabetes or the late stages of type 2 diabetes, insulin replacement therapy is necessary. Insulin therapy, although life-saving, does not restore normoglycemia, even when continuous infusions or multiple injections are used in complex regimes. For example, postprandial levels of glucose continue to be excessively high in individuals on insulin replacement therapy. Thus, insulin therapy must be delivered by multiple daily injections or continuous infusion and the effects must be carefully monitored to avoid hyperglycemia, hypoglycemia, metabolic acidosis, and ketosis.
People with type 2 diabetes are generally treated with drugs that stimulate insulin production and secretion from the beta cells and/or improve insulin sensitivity. A major disadvantage of these drugs, however, is that insulin production and secretion is promoted regardless of the level of blood glucose. Thus, food intake must be balanced against the promotion of insulin production and secretion to avoid hypoglycemia or hyperglycemia. In recent years several new agents have become available to treat type 2 diabetes. These include metformin, rosiglitazone, pioglitazone, and acarbose (see Bressler and Johnson, 1997). However, the drop in hemoglobin A1c obtained by these newer agents is less than adequate (Ghazzi et al., 1997), suggesting that they will not improve the long-term control of diabetes mellitus.
Glucagon-like peptide-1 (GLP-1), a hormone normally secreted by neuroendocrine cells of the gut in response to food, has been suggested as a new treatment for type 2 diabetes (Gutniak et al., 1992; Nauck et al., J. Clin. Invest., 1993). It increases insulin release by the beta cells even in subjects with long-standing type 2 diabetes (Nauck et al., Diabetologia, 1993). GLP-1 treatment has an advantage over insulin therapy because GLP-1 stimulates endogenous insulin secretion, which turns off when blood glucose levels drop (Nauck et al., Diabetologia, 1993; Elahi et al., 1994). GLP-1 promotes euglycemia by increasing insulin release and synthesis, inhibiting glucagon release, and decreasing gastric emptying (Nauck et al., Diabetologia, 1993; Elahi et al., 1994; Wills et al., 1996; Nathan et al., 1992; De Ore et al., 1997). GLP-1 also induces an increase in hexokinase messenger RNA levels (Wang et al., Endocrinology 1995; Wang et al., 1996). GLP-1 is known to have a potent insulin-secreting effect on beta cells (Thorens and Waeber, 1993; Orskov, 1992) and to increase insulin biosynthesis and proinsulin gene expression when added to insulin-secreting cell lines for 24 hours (Drucker et al., 1987; Fehmann and Habener, 1992). In studies using RIN 1046-38 cells, twenty-four hour treatment with GLP-1 increased glucose responsiveness even after the GLP-1 had been removed for an hour and after several washings of the cells (Montrose-Rafizadeh et al., 1994). Thus, GLP-1 is an insulinotropic agent known to have biological effects on βcells even after it has been metabolized from the system. GLP-1 is a product of posttranslational modification of proglucagon. The sequences of GLP-1 and its active fragments GLP-1 (7-37) and GLP-1(7-36) amide are known in the art (Fehmann et al., 1995). Although GLP-1 has been proposed as a therapeutic agent in the treatment of diabetes, it has a short biological half-life (De Ore et al., 1997), even when given by a bolus subcutaneously (Ritzel et al., 1995). GLP-1 degradation (and GLP-1 (7-36) amide), in part, is due to the enzyme dipeptidyl peptidase (DPP1V), which cleaves the polypeptide between amino acids 8 and 9 (alanine and glutamic acid).
Exendin-4 is a polypeptide produced in the salivary glands of the Gila Monster lizard (Goke et al., 1993). The amino acid sequence for exendin-4 is known in the art (Fehmann et al. 1995). Although it is the product of a uniquely non-mammalian gene and appears to be expressed only in the salivary gland (Chen and Drucker, 1997), exendin-4 shares a 52% amino acid sequence homology with GLP-1 and in mammals interacts with the GLP-1 receptor (Goke et al., 1993; Thorens et al., 1993). In vitro, exendin-4 has been shown to promote insulin secretion by insulin producing cells and, given in equimolar quantities, is more potent than GLP-1 at causing insulin release from insulin producing cells. Furthermore, exendin-4 potently stimulates insulin release to reduce plasma glucose levels in both rodents and humans and is longer acting than GLP-1. Exendin-4, however, because it does not occur naturally in mammalians, has certain potential antigenic properties in mammals that GLP-1 lacks.
In addition to the reduction in insulin production that occurs in diabetes, peripheral neuropathy is commonly associated with diabetes. Twenty to thirty percent of all diabetes subjects eventually develop peripheral neuropathy. Furthermore, there are reports of increased risk of Alzheimer's disease with heart disease, stroke, hypertension, and diabetes (Moceri et al., 2000; Ott et al., 1999). Thus, diabetes is a disease that is also associated with neurodegenerative diseases.
A number of studies have demonstrated that the GLP-1 receptor is present in both the rodent (Jin et al 1988, Shughrue et al 1996) and human (Wei and Mojsov 1995, Satoh et al 2000) brains. The chemoarchitecture of the distribution appears to be largely confined to the hypothalamus, thalamus, brainstem, lateral septum, the subformical organ and the area postrema, all circumventricular areas where generally large numbers of peptide receptors are located. However, specific binding sites for GLP-1 have also been detected throughout the caudate-putamen, cerebral cortex and cerebellum (Campos et al. 1994, Calvo et al. 1995, Goke et al. 1995), albeit at low densities.
Needed in the art are polypeptides that are of therapeutic value in the treatment of diabetes and the treatment of degenerative disorders such as Alzheimer's and Parkinson's diseases, as well as the peripheral neuropathy associated with type 2 diabetes mellitus.